Volatage drop calculator



y 1945- H. SCHAEVITZ I 2,379,931

VOLTAGE DROP CALCULATOR Filed March 15, 1944 CABLE SIZE UNIVERSAL VOLTAQDRQP LCULATOR LSE'I PHASE ON APPLIED VOLTAGE. 2.551 POWER FAQTDR SCALET0 CROSS 4.1mm POINTER;

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Application March 15, 1944, Serial No. 526,819

Claims. I ('01. 23584) (Granted under the act of March 3, 1883, asamended April 30, 1928; 370 0. G. 757) This invention relates to avoltage drop calculator and has for an object to provide an improvedvoltage drop calculator which is compact in size and form and isextremely accurate as compared to prior calculators.

A further object of this invention is to provide a voltage dropcalculator which includes all the various factors necessary for accuratecalculations of a voltage drop, including the applied voltage, the powerfactor, the cable size, the line current, the cable length, and finallythe desired result of the percent of voltage drop.

A further object of this'invention is to provide a voltage dropcalculator which eliminates the use of charts or slide rules and themanipulation of a straight edge with such charts and which enables thefinal result to be calculated directly from the various factors on thisone device.

With the foregoing and other objects in view, one form of the inventionconsists in the construction, combination and arrangement of the partshereinafter described and illustrated in the drawing, in which:

Fig. 1 is a plan view of the voltage drop calculator of this invention.

Fig. 2 is a sectional view on line 22 of Fig. 1, and

Fig. 3 is an enlarged sectional detail view through the center of thedevice.

There is shown at In the voltage drop calculator of this invention. Thisincludes a circular base disc II on which is printed or engraved thepercent voltage drop I! along about one-half of its periphery, while theother half of this base disc H is provided with cable size curves shownat I3. Rotatable over the center of the base disc II on a, stepped pivotbolt [4 is a semi-circular length-or-line current disc l5 of somewhatsmaller diameter than the base disc ll. above the semi-circular disc I5is pivoted a power factor disc l8 which includes a circular portion llof substantially smaller diameter than the disc IS. A power factorpointer l8 extends from disc Is to a distance Slightly less than theouter periphery of the length-or-line current disc I5. Above this ispivoted a transparent pointer 20 which, it will be observed from Fig. 2,is offset at 2| and 22 to lie snugly against the discs l5 and II. Thepointer 20 is of a transparent material and is provided with a pointerhair line 23 extending radially from the center of the pivot to the edgeof the base disc ll so as to enable the various factors to be aligned inthe calculation.

Next 4 The discs II, It and I! are of any suitable material, such ametal, plastic, cardboard, etc.

The stepped pivot bolt I4 is provided with a circular boss 24 of aslightly greater thickness than the thickness of the base disc II, whilethe disc II is provided with a pivot opening 25 of a diametercorresponding to the diameter of the boss 24, permitting it to revolvefreely thereon. The bolt I4 is provided with a center shank 26connecting it to the threaded portion 21 for reception of a steppedknurled nut 28. The discs 15 and I6 have .pivot openings correspondingto the diameter of the shank 26, while the pointer 20 has a pivotopening corresponding to the diameter of a stepped boss 30 on nut 28,the boss 30 being of greater length than the thickness of the pointer20. A head 31 on bolt l4 provides a support for the bottom of the basedisc H. As a result of this construction, particularly shown in Fig. 3,it will be observed that the discs l5 and It may be locked to each otherby tightening the nut 28 for rotation as a unit, while leaving the basedisc II and pointer 20 free to rotate independently of the locked discsl5 and I6.

The formula which is used for calculating voltage drops is:

CI (FL) A-V Percent voltage drop= XD. F.X%

where:

C=resistlvity of copper=10.8 ohms per circular mil area, one foot inlength.

==balanced current in each conductor, in amperes.

F=factor depending upon type of current and number of phases; F=2 fordirect current or single phase alternating current; F=1.732 for threephase alternating current.

L=length, in feet of one conductor.

A=area of each conductor, in circular mils.

V=impressed voltage, line to line.

D. F.=ratio of.voltage drop in the cable (input voltage minus outputvoltage) to the resistive voltage drop. This factor depends on conductorsize, stranding and spacing, type of surrounding medium (i. e., magneticor non-magnetic), and load power factor.

In designing the voltage drop calculator l0, values of percent voltagedrop H were calculated by the above formula and imposed on thecalculator ID by reverse procedure to produce the cable curves 13.Assuming a constant current of 100 amperes, and a constant cable lengthof 100 feet, values were computed for an applied voltage 32 of 450volts, three phase. These values were obabove all, accuracy.

the utility desired, the form of the calculator herein described wasproduced in the following manner. A logarithmic scale of two cyclesranging from 0.1 to 10 was first marked out on 180 degrees of the outerrim of the disc H. A companion logarithmic scale of 180 degrees extentof two cycles ranging from 10 to 1000 was next applied to the rim of thehalf disc IS. Th two scales progress clockwise and are designated i 2and 33 respectively. A logarithmic scale 32 ranging from 100 to 500 wasmarked, progressing clockwise, on the half disc iii. The exact location,circumferentially or radially, of this scale is a matter of convenience.In the herein described calculator it was placed about half-way alongthe radius of the half disc ii. The mechanical center of the scal 32 wasplaced on approximately the index point 35 of scale 33. The angularadvance of scale 32 is equal to that of scale 33 for correspondinglogarithmic increments. The marks 33 on disc l6 are two points on acompanion logarithmic scale to scale 32. They correspond to the values:2 for direct current or single phase alternating current, and 1.732 forthree phase alternating current, of the factor F in the previouslydiscussed "percent voltage drop" formula. Scale 35 was marked on arm l8so that when the 3 mark of scale 35 coincided with 450 volts on scale32, scale 33, hereinafter to be described, was on the extension of theradius on which the index point 36 was located. A uniformly graduatedlinear scale 34, of arbitrary length and radial location was marked onthe arm I 8. The range from 40% to 100% was used since this range isthat most useful in calculation of voltage drops.

Although the curves I3 could now be plotted from the percent voltagedrop equation above if that equation were transformed to its logarithmicform, the fact that it contains the drop factor makes this inadvisablesince this factor does not vary linearly with power factor or cablesize. As described in the third paragraph above, values of percentvoltage drop were calculated by means of the above formula, and a set ofdrop factor tables, for various combinations of load, power factor andcable size.

The mechanical design of the calculator ill and the arrangement ofscales were developed by trial to achieve compactness, relativesimplicity, and The point marked I on the "feet-ampere scale 33corresponds to 100 feet and 100 amperes simultaneously. Thus, by settingthis point on a value of voltage drop calculated by the above formula, apoint on a cable curve I3 may be plotted under the power factor forwhich the value was computed. By moving the index I00 point 33 to all ofthe values calculated for various combinations of power factor and cablesize, the complete set of curves is plotted.

It is seen from the above that this calculator does not depend on theproportional variation of its indicated result with all of the factorsinvolved in this result. Thus the variation of voltage drop with variousfactors can be obtained empirically and placed upon this calculator inspite of the non-logarithmic correspondence of this drop with cable sizeor power factor.

In using the derived curves to furnish a value of percent voltage drop,the power factor scale 34 on pointer I3 is moved until the chosen powerfactor is over the chosen cable size'13 (in thousands of circular mils).If the positioning thumb screw it is locked with the three phase point33 on the top disc I! over 450 volts on the middle disc II. the indexI00 point 33 will be over the percent voltage drop i2 for 450 volts,three phase applied to a cable of chosen size feet long supplying 100amperes to a load at the chosen power factor. If either or both currentand length differ from 100, the feetampere scale 33 and the percentvoltage drop scale I 2 are used as ordinary logarithmic scales inmultiplication.

Since the number of phases and. the input voltage enter as factors bywhich the voltage drop is varied, and since they usually remain fixed ina series of computations, they may be incorporated as logarithmicvariations of the relative position of the feet-ampere scale 33 withrespect to the power factor scale 34. Thus, for a fixed position of thepower factor scale 34 th index loo point 36 will vary its position inaccordance with the logarithmic change due to varying input voltage ornumber of phases.

In operation, first, the phase scale 35 is set on the applied voltagescale 32, and the nut 23 is tightened to lock discs I5 and it together.Then the power factor scale 33 is set to cross the proper cable size 13at the load power factor. If for direct current, use the power factorequal to 100. Then the hair line 23 of the pointer 20 is set on thelength scale 33. Next, holding the end of the pointer 20 tight againstdisc ii, move the line current scale 33 on disc i5 until the 100 point33 is under the pointer hair line 23. Then set the pointer line 23 onthe line current along the scale 33. New read the percent of voltagedrop on the scale l2 as it appears under the pointer line 23 which willbe the final result desired.

The following are specific examples showing how problems are solved withthe calculator of this invention:

Problem 1.'I'HFA-9; length of single conductor, 154. feet; balancedthree phase current, 24 amperes; applied voltage, 450 volts. (Findvoltage drop in cable.)

Procedure: Loosen pivot screw, set 3 5 on 450 volts, tighten pivotscrew; place 0.74 power factor over curve for 9 cable size; move pointerto position over 24 amperes, hold pointer against back plate, moveclamped assembly until 100 on scale 33 is under index line of pointer;move pointer only to 154 feet on length scale.

Answer: Find 1.58% voltage drop under index line.

Problem 2.--THFA40; power factor, 0.60; direct current, 50 amperes;applied voltage, 240 volts. (Find maximum length of cable permissiblefor 5% voltage drop.)

Procedure: Loosen pivot screw, set D. C. on 240 volts, tighten pivotscrew; place 0.60 power factor over curve for 40 cable size; move indexline over 50 amperes, hold pointer against back plate, move clampedassembly until 100 on scale 33 is under index line on pointer; movepointer only to 5% voltage drop.

Answer: Find 492 feet under index line.

Problem 3.-I.ength of cable run, 250 feet; single phase current, 25amperes; applied voltage, 117 volts; power factor, 1.0. (Find size ofcable required to keep voltage drop under 8%).

Procedure: Loosen pivot screw, set 14: on 117 volts, tighten pivotscrew; set 250 feet over 8% voltage drop; move index line on pointer to100 on feet scale; hold pointer against back plate, move clampedassembly until 25 amperes is under pointer index line.

power factor, 0.74

factor disc adjacent its periphery,

Answer: Find 1.0 on power factor scale between 14 and 23 cable curves.This indicates that THFA-23 cable is required.

Problem 4.--THFA300; power factor 0.80; length of single conductor, 200feet; applied voltage 400 volts. (Find permissible 34 current, ifvoltage drop is to be limited to 2%.)

Procedure: Loosen pivot screw, set 3 over 400 volts, tighten pivotscrew; place 0.80 power factor over curve for 300 cable size; movepointer index line over 200 feet, hold pointer against back plate, moveclamped assembly until 100 on length scale is under pointer index line;move pointer over 2% voltage drop.

Answer: Find 460 amperes under index line.

Other modifications and changes in the proportions and arrangements ofthe parts may be made by those skilled in the art without departing fromthe nature and scope of the invention, as defined in the appendedclaims.

The invention described herein may be manuiactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

What is claimed is:

1. In a voltage drop calculator of the character described, a circulardisc having a series of cable size curves extending somewhat radiallyover about a semi-circular half of said disc, 9, logarithmic scale ofpercent voltage drop extending along the periphery of the other half ofsaid disc, 9. semi-circular disc of slightly smaller diameter rotatableover said first mentioned disc and hearing a logarithmic scale of lengthin feet or line current in amperes, said logarithmic scalecorrespondingto the first mentioned logarithmic scale, a smaller dischaving a power factor scale pointer extending radially therefrom andadapted to cooperate with said above mentioned cable size curves, aphase scale on said power factor disc adjacent its periphery, and anapplied voltage scale on said semi-circular disc adjacent the peripheryof said power factor disc to cooperate with said phase scale thereon,and a pointer line member pivoted to rotate over said three discs.

2. In a voltage drop calculator of the character described, a circulardisc having a series of cable size curves extending somewhat radiallyover about a semi-circular half of said disc, a logarithmic scale ofpercent voltage drop extending along the periphery of the other half ofsaid disc, a semi-circular disc of slightly smaller diameter rotatableover said first mentioned disc and bearing a logarithmic scale of lengthin feet or line current in amperes, said logarithmic scale correspondingto the first mentioned logarithmic scale, a smaller disc having a powerfactor scale pointer extending radially therefrom and adapted tocooperate with said above mentioned cable size curves, a phase scale onsaid power and an applied voltage scale on said semi-circular discadjacent the periphery of said power factor disc to cooperate with saidphase scale thereon, a pointer line member pivoted to rotate over saidthree discs, and means for pivoting said discs and pointer togetherincluding means for locking said semi-circular disc and said powerfactor disc for unitary rotation.

3. In a voltage drop calculator of the character described, a circulardisc having a series of cable size curves extending somewhat radiallyover about a semi-circular half of said disc, a logarithmic scale ofpercent voltage drop extending along the periphery of the other half ofsaid disc, at semi-circular disc of slightly smaller diameter rotatableover said first mentioned disc and bearing a logarithmic scale of lengthin feet or line current and amperes, said logarithmic scalecorresponding to the first mentioned logarithmic scale, a still smallerdisc having a power factor scale pointer extending radially therefromand adapted to cooperate with said above mentioned cable size curves, aphase scale on said po er factor disc adjacent its periphery,

and an app ied voltage scale on said semi-circular disc adjacent theperiphery of said power factor disc to cooperate with said phase scalethereon, a transparent pointer line member pivoted to rotate over saidthree discs, and means for pivoting said discs and pointer togetherincluding means for locking said semi-circular disc and said powerfactor disc for unitary rotation, said locking means including a steppedpivot bolt and a stepped pivot, nut, said circular disc and said pointermember being rotatable on steps of said stepped bolt and of said steppednut, said semicircular and power factor discs being rotatable on theshank of said bolt and adapted to be locked between the steps of saidbolt and said nut.

4. A voltage drop calculator comprising a disc having delineated thereona semi-circular logarithmic scale for percent voltage drop and asemi-circular grouping of cable size curves, 3. semi-circular discpivotally mounted thereover having a peripheral logarithmic scaledelineated thereon for length or line current, said logarithmic scalescorresponding to each other, a power factor disc pivotally mountedthereover having a radially extending power factor scale cooperativewith said power factor curves on said circular disc, a phase settingscale on the disc of said power factor pointer and an applied voltagescale on said semi-circular disc adapted to cooperate with said phasesetting scale on said power factor disc and a transparent pointer armhaving a radially extending hair line index there- 5. A voltage dropcalculator comprising a disc having delineated thereon a semi-circularlogarithmic scale for percent voltage drop and a semi-circular groupingof cable size curves, a semi-circular disc pivotally mounted thereoverhaving a peripheral logarithmic scale delineated thereon for length orline current, said logarithmic scales corresponding to each other, apower factor disc pivotally mounted thereover having a radiallyextending power factor scale cooperative with said cable size curves onsaid circular disc, a phase setting scale on the disc of said powerfactor pointer and an applied voltage scale on said semi-circular discadapted to cooperate with said phase setting scale on said power factordisc and a transparent pointer arm having a radially extending hair lineindex thereon, said pointer arm being stepped to closely over.- lie thescales of said disc and stepped bolt and nut means for pivoting saiddiscs and arms freely and for locking said semi-circular and powerfactor discs for unitary rotation relative to said circular disc andsaid pointer arm.

HERMAN BCHAEVITZ.

